1. Field of the Invention
The present invention pertains generally to the field of telescopically extensible supports and in particular relates to improved leg locking devices for telescopically extensible tripod legs.
2. Description of the Prior Art
Photohgraphic tripods have long taken advantage of telescopically extensible legs for convenience and portability and also for the inherent possibility of raising the camera to different heights by extending the tripod legs to varying degrees. Further, the various legs can be adjusted in length to make up for irregularities in the ground or other underlying surface. Conventional tripod legs are made of two or more leg sections of similar cross-section but of progressively diminishing cross-sectional dimension so that the largest section can telescopically receive all subsequent, smaller leg sections. Each section except for the lowest and thinnest leg section is provided with a manual locking device for interlocking each leg section to the immediately following leg section i.e. the next smaller section, so that once the leg is extended to a desired overall length, it can be locked for supporting some weight without collapsing, and the telescoping sections together form a reasonably rigid support for the photographic device or other apparatus mounted to a tripod head. Each leg lock is associated with an outer leg section and an inner leg section. Two such leg locks are normally found in a typical three-section tripod leg.
One type of previously used leg lock includes a locking lever which is pivotally mounted by means of a bracket to the outer leg section and includes a cam element which, in a locking position of the lever, presses against the inner leg section through a window cut-out in the outer leg section, so that the inner section is pressed against an inner surface of the outer leg section opposite the window and the two leg sections are frictionally retained against further relative telescopic displacement. A pressure pad is normally disposed within the window opening between the cam element and the inner leg section for better distributing the cam pressure against the inner leg section.
This basic locking principle has been adopted in various forms and in different arrangements and configurations of the locking lever and the cam element.
A chief consideration in the design of this type of leg lock is the prevention of denting or deformation of the inner leg section, which is typically made of lightweight aluminum tubing for portability, under pressure of the locking lever. Since the aluminum tubing is not resilient, once deformed it may happen that the cam lever is no longer able to apply sufficient locking force at the deformed site and effective locking of the leg support is no longer possible. Other important considerations include the long term dependability of the cam element by avoiding deformation, wear or loss of elasticity of the cam element after many locking operations, so that cam pressure may be reliably and repeatedly applied to the inner leg section. Ease of operation of the locking lever is also important i.e. that relatively little force be required to move the lever into locking position or release the lever from this position.
The pressure pad is typically held captive within the window opening between the cam surface of the locking lever and the outer surface of the inner leg section. Because of the pivotal movement of locking lever it is necessary to accomodate some movement of the cam surface relative to the pressure pad, either by a resilient yielding of the pad, or to provide for cam-following-movement of the pad as the locking lever is pivoted between locking and released positions. This in turn places certain restrictions on the type of tubing which can be used for the leg sections. It is particularly desirable to provide for pivotal movement of the locking lever transverse to the leg sections, which is considerably more convenient from a user's stand point. This in turn dictates that the pressure pad must move with the cam surface against the surface of the inner leg section transversely to the leg axis. This calls for a smooth outer surface of the leg section as opposed to a longitudinally corrugated, reinforced leg section which is considerably stronger and more resistant to indentation or deformation under pressure of the locking lever.
Further improvement in such leg locking devices is therefore desirable to assure positive locking of the telescoping leg, easy lever action and dependable long term operation of the lock with a transversely pivoted lever.